1. Field of the Invention
The present invention relates generally to optical lithography, and particularly to optical microlithography crystals for use in optical photolithography systems utilizing vacuum ultraviolet light (VUV) wavelengths below 193 nm, preferably below 175 nm, more preferably below 164 nm, such as VUV projection lithography refractive systems utilizing wavelengths in the 157 nm region.
2. Technical Background
Semiconductor chips such as microprocessors and DRAM""s are manufactured using a technology called xe2x80x9cOptical Lithographyxe2x80x9d. An optical lithographic tool incorporates an illuminating lens system for illuminating a patterned mask, a light source and a projection lens system for creating an image of the mask pattern onto the silicon substrate.
The performance of semiconductors have been improved by reducing the feature sizes. This in turn has required improvement in the resolution of the optical lithographic tools. In general, the resolution of the transferred pattern is directly proportional to the numerical aperture of the lens system and inversely proportional to the wavelength of the illuminating light. In the early 1980""s the wavelength of light used was 436 nm from the g-line of a mercury lamp. Subsequently the wavelength was reduced to 365 nm (I-line of mercury lamp) and currently the wavelength used in production is 248 nm obtained from the emission of a KrF laser. The next generation of lithography tools will change the light source to that of an ArF laser emitting at 193 nm. The natural progression for optical lithography would be to change the light source to that of a fluoride laser emitting at 157 nm. For each wavelength different materials are required to fabricate lenses. At 248 nm the optical material is fused silica. For 193 nm systems there will be a combination of fused silica and calcium fluoride lenses. At 157 nm fused silica does not transmit the laser radiation. At present the only material deemed to be useable at 157 nm is calcium fluoride. See, for example, Japanese unexamined Patent Publication No. 10-13 10 of Jan. 6, 1998 from Application No. 8153769 filed Jul. 14, 1996 and Japanese Patent No. 10-59799 Laid Open Mar. 3, 1998 based on an application filed Aug. 20, 1996, the Applicant in both cases being the NIKON Corporation.
Projection optical photolithography systems that utilize the vacuum ultraviolet wavelengths of light below 193 nm provide benefits in terms of achieving smaller feature dimensions. Such systems that utilize vacuum ultraviolet wavelengths in the 157 nm wavelength region have the potential of improving integrated circuits with smaller feature sizes. Current optical lithography systems used by the semiconductor industry in the manufacture of integrated circuits have progressed towards shorter wavelengths of light, such as the popular 248 nm and 193 nm wavelengths, but the commercial use and adoption of vacuum ultraviolet wavelengths below 193 nm, such as 157 nm has been hindered by the transmission nature of such vacuum ultraviolet wavelengths in the 157 nm region through optical materials. For the benefit of vacuum ultraviolet photolithography in the 157 nm region such as the emission spectrum VUV window of a F2 excimer laser to be utilized in the manufacturing of integrated circuits there is a need for optical lithography crystals that have beneficial optical properties below 164 nm and at 157 nm.
The present invention overcomes problems in the prior art and provides a fluoride optical lithography crystal that can be used to improve the manufacturing of integrated circuits with vacuum ultraviolet wavelengths.
One aspect of the present invention is a below 160 nm optical lithography barium fluoride crystal. The barium fluoride crystal has a refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003 at 157 nm.
In another aspect, the present invention includes a dispersion management optical lithography crystal. The dispersion management crystal is an isotropic barium fluoride crystal. Preferably the barium fluoride crystal has a 157.6299 nm refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003 and a 157.6299 nm refractive index n greater than 1.56.
In a further aspect, the present invention includes a below 160 nm optical lithography method which comprises providing a below 160 nm optical lithography illumination laser, providing a calcium fluoride crystal optical element, providing a barium fluoride crystal optical element having a below 160 nm refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003, and transmitting the below 160 nm optical lithography light through the calcium fluoride optical element and the barium fluoride optical element to form an optical lithography pattern.
In another aspect, the present invention includes a method of making a dispersion managing optical lithography element. The method includes providing a barium fluoride source material, melting the barium fluoride source material to form a precrystalline barium fluoride melt, solidifying the barium fluoride melt into a barium fluoride crystal and annealing the barium fluoride crystal to provide an isotropic barium fluoride crystal with a 157 nm refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the invention as described herein, including the detailed description which follows and the claims.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview of framework for understanding the nature and character of the invention as it is claimed.
Wherein reducing the wavelength of the illuminating light for lithography processes is necessary to achieve higher resolution, the illuminating light laser emission has a finite bandwidth. To achieve the resolution required at the 100 nm node, the optical lithography tool manufacturer using an all refractive optical design can either use a very highly line narrowed laser (to less than 2 pm) or can use two optical materials which have dispersion properties that compensate for the bandwidth of the laser.
In a preferred embodiment the invention includes providing isotropic optical lithography crystalline materials for color correction for VUV lithography in general but especially in the region of 157 nm to enable refractive lenses to be constructed to make use of the light from a fluoride excimer laser that has not been line narrowed to below 2 pm. The invention includes a range of fluoride crystalline materials that provide benefits to 157 nm optical lithography. In the preferred embodiment the dispersion managing optical lithography crystal is utilized in conjunction with a 157 nm optical lithography illumination laser with a bandwidth not less than 0.2 picometers.
The refractive index of a material varies with the wavelength of energy passing through it and this is called the dispersion of the material. Hence if light passing through a lens system, constructed of one optical material, has a range of wavelengths then each wavelength would be brought to a different focus so reducing resolution. This effect can be overcome by using a second optical material with different dispersion characteristics. This technique is known as color correction. To be of use as a color correction material, there are specific criteria that have to be met, namely the material must transmit at the wavelength of operation, it must be isotropic and must have optimum dispersion characteristics. At 157 nm, the only material that has had its dispersion characteristics measured to the necessary degree of accuracy is calcium fluoride. Applying the criteria of 157 nm transmission and of being isotropic, the following materials can be used as color correction materials.
I. Materials Based on Alkali Metal Fluorides
Lithium fluoride, sodium fluoride, potassium fluoride, and materials of the formula: MRF3 in which M is either Li, Na or K and R is either Ca, Sr, Ba or Mg. Examples of such materials include but are not limited to: KMgF3, KSrF3, KBaF3, KCaF3, LiMgF3, LiSrF3, LiBaF3, LiCaF3, NaMgF3, NaSrF3, NaBaF3, and NaCaF3.
II. Materials Based on Alkaline Earth Metal Fluorides
Calcium fluoride, barium fluoride and strontium fluoride. Each of these materials can be combined with the other to form a mixed crystal of the formula (M1)x(M2)1xe2x88x92xF2 in which M1 is either Ba, Ca or Sr and M2 is either Ba, Ca or Sr and x is an quantity between 0 and 1. Non-limiting examples are Ba0.5Sr0.5F2 in which x=0.5 and Ba0.25Sr0.75F2, in which x=0.75. When x=0, the materials are CaF2, BaF2, SrF2 
III. Materials Based on Mixed Crystals of the Formula M1xe2x88x92xRxF2+1, in which M is either Ca, Ba or Sr and R is lanthanum.
In such materials the structure of the crystal is isotropic up to x values of 0.3. Examples of this formula include but are not limited to Ca0.72La0.28F2.28 in which x=0.28 or Ba0.74La0.26F2.26 in which x=0.26 or of the type Sr0.79La0.21F2.21 in which x=0.21.
Each of the above materials I, II and III can be manufactured using a technique known as the xe2x80x9cStockbargerxe2x80x9d or xe2x80x9cBridgmanxe2x80x9d technique of crystal growth. This process comprises loading the powder of the material to be grown into a container known as a crucible. The crucible which usually is made of high purity graphite is positioned on a moveable support structure within a heater with sufficient power to raise the temperature to a level above the melting point of the material to be grown. After assembling the heater system around the crucible, the system is closed with a bell jar and evacuated using a combination of vacuum pumps. After a vacuum exceeding 10xe2x88x925 Torr has been achieved, power is applied to the heater and is continually raised until a preset level has been achieved. This preset level of power is defined by melt trial runs. After a period of several hours at the melt power, the moveable support structure is activated and the crucible is made to slowly descend in the furnace. As the tip of the crucible descends, it cools and the molten material begins to freeze. By continuing the descent there is progressive solidification until all the melt is frozen. At this point, the power to the furnace is reduced to below the melt power, the crucible is raised back into the heater, allowed to reach thermal equilibrium over a period of several hours and then allowed to cool to room temperature by slowly reducing the power to the heaters. Once at room temperature, the vacuum is released, the bell jar removed followed by the heaters and the crystal can be removed from the crucible.
Although the Bridgman or Stockbarger method of crystal growth is the usual method of growing crystals of fluoride based materials, it is not the only method available. Techniques such as the Czochralski, Kyropoulos or Stober methods can also be utilized.
The size and shape of the disks resulting from these materials are variable e.g. for lenses: 118-250 mm in diameter by 30-50 mm in thickness. The disks are ground in a conventional manner to lenses of about the same dimensions and having the desired curvature. The lenses have a general application, for example, whenever color correction is required. The lenses can then be incorporated in a wide variety of optical systems, e.g. lasers including but limited to the 157 nm systems, spectrography systems, microscopes and telescopes.
The preceding specific embodiments are to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.
The entire disclosure of all applications, patents and publications, cited above and below, are hereby incorporated by reference.
From the foregoing description one skilled in the art can easily ascertain the essential characteristics of this invention, and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.
The invention includes a below 160 nm optical crystal transmitting for use with a below 160 nm lithography laser having a bandwidth of at least 0.2 pm. The optical lithography crystal is comprised of an isotropic barium fluoride crystal which has a 157 nm transmission greater than 85% and a refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003 at 157 nm. Preferably the barium fluoride crystal has a 157 nm, refractive index wavelength dispersion dn/dxcex less than xe2x88x920.004, and more preferably  less than xe2x88x920.0043. Preferably the barium fluoride crystal has a 157 nm refractive index index n greater than 1.56. More preferably nxe2x89xa71.6, and most preferably nxe2x89xa71.64. Preferably the barium fluoride crystal has a 157 nm refractive index temperature coefficient dn/dt greater than 8xc3x9710xe2x88x926/xc2x0 C., and more preferably dn/dtxe2x89xa78.5xc3x9710xe2x88x926/xc2x0 C. In a preferred embodiment the optical lithography barium fluoride crystal has a large dimension diameter  greater than 100 mm and a thickness  greater than 30 mm, and more preferably a diameter in the range of about 118 to 250 nm and a thickness in the range of about 30 to 50 mm. When utilized with a broadband width illumination source such as an F2 excimer laser with a bandwidth of at least 0.5 pm, said barium fluoride crystal comprises a bandwidth dispersion managing optical element. Preferably the barium fluoride optical lithography crystal has a sodium contamination content  less than 10 ppm by weight, more preferably  less than 5 ppm by weight, and most preferably  less than 1 ppm. Preferably the barium fluoride optical lithography crystal has a total rare earth contaminant content of less than 1 ppm by weight. Preferably the barium fluoride optical lithography crystal has a total oxygen contaminant content of less than 50 ppm by weight, and more preferably  less than 20 ppm. Such low contaminant levels provide beneficial optical characteristics, and preferably the crystal has a 157 nm transmission xe2x89xa786%, and more preferably xe2x89xa788%.
In a further aspect the invention includes a below 160 nm dispersion management optical lithography crystal. The dispersion management optical lithography crystal comprises an isotropic barium fluoride crystal having a 157.6299 nm refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003 and a 157.6299 nm refractive index n greater than 1.56. Preferably the dispersion management crystal""s dn/dxcex less than xe2x88x920.004, and more preferably dn/dxcex less than xe2x88x920.0043. Preferably the crystal""s refractive index n greater than 1.6.
In a further aspect the invention includes a below 160 nm optical lithography method which includes providing a below 160 nm optical lithography illumination laser, providing a calcium fluoride crystal optical element, and providing a barium fluoride crystal optical element having a below 160 nm refractive index wavelength dispersion dn/dxcex which is  less than xe2x88x920.003. The method includes transmitting below 160 nm optical lithography light through the calcium fluoride optical element and the barium fluoride optical element to form an optical lithography pattern, preferably with feature dimensions xe2x89xa6100 nm. Providing the barium fluoride crystal optical element preferably includes loading a barium fluoride crystal feedstock into a container, melting the feedstock to form a precrystallline barium fluoride melt, and progressively freezing the barium fluoride melt into a barium fluoride crystal. The method preferably further includes heating the barium fluoride crystal and slowly thermal equilibrium cooling the crystal and forming the barium fluoride crystal into an optical element. Preferably the illumination laser has a bandwidth xe2x89xa70.5 pm, and preferably xe2x89xa71 pm. In another aspect the invention includes the method of making a dispersion managing optical lithography element. The method includes providing a barium fluoride source material, melting the barium fluoride source material to form a precrystalline barium fluoride melt, solidfying the barium fluoride melt into a barium fluoride crystal, and annealing the barium fluoride crystal to provide an isotropic barium fluoride crystal with a 157 nm refractive index wavelength dispersion dn/dxcex less than xe2x88x920.003. The method preferably includes providing a contaminant removing fluoride scavenger and melting the scavenger with the barium fluoride source material to remove contaminants. Preferably the scavenger is lead fluoride.